X-ray-absorption fine structure determination of pressure-induced bond-angle changes in ReO3

We report here on a Marquardt-type method to fit the x-ray absorption fine structure (XAFS) of ReO3. We find that, when the ambient-pressure structure of ReO3 is used as a starting point, the pressure dependence of the angle of the Re-O-Re bond in ReO3 is fairly straightforwardly and robustly determined using FEFF curved-wave, multiple-scattering programs and is accurate to about +/-1.5 degrees or better. We present an argument that XAFS and scattering experiments fundamentally differ in what they measure in the case of nearly linear atomic bridges. Focussed multiple-scattering paths involving the Re-O-Re bridge make a contribution to the XAFS spectrum that is sensitive to the rns deviation of oxygen from the [100]-type directions. Fits to simulated spectra back up our contention that for XAFS experiments the effective position of the oxygen atom is its rms displacement whether the average displacement is zero or not.